Host defense against pulmonary pathogens requires the generation of effective T cell directed immune responses. T cell activation is a multi-step process that involves the coordinate participation of adhesion receptors. These interactions regulate lymphocyte trafficking to secondary lymphoid organs and to sites of inflammation, as well as the cognate interactions required for T cell recognition of antigen. The sialoglycoprotein CD43 is among the most highly expressed proteins on the surface of the T cell. CD43 has been implicated in regulating several aspects of T cell biology, including proliferation, adhesion and survival. The most striking aspect of CD43 is its ability to alter cell adhesion, which is likely the primary function of CD43. The mechanism by which CD43 regulates the adhesive properties of the T cell has been proposed to be a function exclusively of the structure of the extracellular domain. The ectodomain of CD43 is heavily glycosylated and sialylated, and as such has a rigid conformation that extends outward from the cell membrane a distance far greater than the T cell receptor. This has led to the predominant model of CD43 function, that it sterically hinders the ability of integrins and other cell surface receptors to bind ligand.Our studies demonstrate 2 critical findings that fundamentally alter the way in which CD43 must be considered. First, we have determined the cytoplasmic domain is both necessary and sufficient for the regulation of cell adhesion. Thus, the steric hindrance model is no longer a tenable explanation for how CD43 regulates cell adhesion. Second, we demonstrate that the cytoplasmic tail of CD43 contains a nuclear localization signal that can mediate translocation of the intracellular domain into the nucleus, and that CD43 can associate with known nuclear proteins. Thus, we hypothesize that the mechanism by which CD43 regulates cell adhesion is based upon specific interactions with intracellular signaling pathways, and involves a novel mechanism in which the intracellular domain translocates to the nucleus and regulates gene transcription. In this application we propose 3 specific aims to determine the mechanism by which CD43 regulates T cell function. First, we will determine the specific element(s) within the intracellular domain of CD43 that regulate cell adhesion. Both in vitro and in vivo systems will be utilized to study the effects 01 specific mutations on the adhesive properties of the cell. Second, using both a genetic and proteomic approach, we will determine the proteins that interact with the intracellular domain of CD43. This data will identify the pathways by which CD43 exerts its regulatory function. Third, we will determine how CD43 interfaces with known cytoskeletal and signaling pathways that regulate cell adhesion, and explore the novel observation that the intracellular domain of CD43 may enter the nucleus and directly alter gene transcription. The successful completion of these studies will increase our understanding of how T cells function, and suggest novel strategies to alter the outcome of T cell mediated diseases.